Moldable thermoplastic polyester crystalline resin blends offer a high degree of surface hardness, solvent resistance and abrasion resistance, high gloss, and low surface friction. However poor impact strength and limited resistance to hydrolytic breakdown can limit usefulness in some application where articles made from the polyester will be exposed to hot wet conditions.
Often a rubbery modifier is added to polyesters to improve impact strength. For example, improved impact strength is obtained by melt compounding polybutylene terephthalate with ethylene homo- and copolymers functionalized with either acid or ester moieties as taught in U.S. Pat. Nos. 3,405,198; 3,769,260; 4,327,764; and 4,364,280. Polyblends of polybutylene terephthalate with a styrene-alpha-olefin-styrene triblock are taught in U.S. Pat. No. 4,119,607. U.S. Pat. No. 4,172,859 teaches impact modification of polybutylene terephthalate with random ethylene-acrylate copolymers and EPDM rubbers grafted with a monomeric ester or acid functionality. However addition of rubbery modifiers can case other issues such as poor melt stability, reduced melt flow, lower modulus and reduced color stability when exposed to heat.
The issue of improving polyester hydrolytic stability has been addressed through use of additive such as carbodiimides as described in U.S. Pat. No. 3,193,522 and epoxy compounds as described in U.S. Pat. No. 5,596,049.
However a balance of performance wherein the polyester resin have good impact and elongation, and are resistant to loss of impact and elongation under wet and dry conditions, while showing limited color change has been difficult to achieve. Thus there exists a need for crystalline polyester compositions with good melt processability, high solvent resistance good impact and elongation and low color which retains these features when exposed to heat.
The abstract of U.S. Pat. No. 6,187,848 to Pixton et al states: “A lightly colored flame retarded reinforced polyester molding composition has color stability at relatively high temperatures and comprises (a) from 40 to 80 weight percent of a polyester resin; (b) from about 20 to about 50 weight percent fibrous glass; (d) from 2.5 to 20 weight percent of a flame retarding composition; (e) and less than about 5 percent by weight additional ingredients which include a combination of stabilizers consisting essentially of an acid quencher, a thioester and a phosphonite.” In this case impact modifiers are detrimental to thermal color stability. Also the color stable compositions of Pixton do not use hindered phenol antioxidants since, as indicated by R. Gauchter and H. Muller in “Plastics Additives Handbook” 4th Ed. Hanser Publishers, p 31, hindered phenol antioxidants can cause yellowing
U.S. Pat. No. 5,300,546 to Walsh relates to polyester compositions with mineral fillers giving a ceramic feel which have improved hydrolytic stability and melt viscosity stability. As set forth in column 2, lines 65 to column 3, line 2, “a thermoplastic resin composition comprising (a) a polyester resin; (b) a mono- or polyepoxy compound; and (c) mineral filler” is described. Additional ingredients are described as “(d) a catalyst compound and/or (e) a hindered phenol antioxidant.” While describing improved hydrolysis resistance in highly filled polyester compositions using the epoxide additives similar to U.S. Pat. No. 5,596,049, this case does not address the question of improved impact, thermal aging and thermal color stability.
A problem with hydrolytically stabilized polyester blends is the lack of impact and the heat stability of the impact modified composition. Poor heat stability may result in discoloration and mechanical failure of a molded article. Loss of heat stability may be evidenced by a decrease in impact resistance and a shift in the coloration. It is desirable to improve heat stability and color stability without deleterious affecting the hydrolytic stability of the polyester composition. It is also important in many applications requiring impact resistance to achieve hydrolytic stability and color stability along with retention of impact and elongation. Accordingly, there is a need for enhancing the impact, as well as the retention of impact and color upon thermal aging in a hydrolytically stable polyester resin.